In the eye, the final stage of image processing in the retina is performed by retinal ganglion cells. The axons of the ganglion cells project out of the eye to form the optic nerve. Glaucoma, which produces irreversible blindness if not treated early enough, destroys these ganglion cells. A typical "early" sign of glaucoma is the loss of a portion of the peripheral visual field, referred to as "scotoma". Unfortunately, by the time a scotoma is detected, the disease has reached a stage where treatment is unlikely to be successful. Another indication of glaucoma is the presence of "cupping" of the optic disc.
One approach that has been used to detect glaucoma at an earlier stage than when a scotoma has developed is to test the intraocular tension of a patient. A symptom of glaucoma is the increase of intraocular tension. Tests of intraocular tension, however, usually involve the use of drugs, are time consuming, and are unpleasant for the patient. Moreover, in the early stages of glaucoma, an increased intraocular tension occurs only transiently in the morning and in the evening, and the susceptibility of patients to pressure damage as a result of increased intraocular tension varies. Moreover, some glaucoma patients do not exhibit intraocular tensions above about 21 mm of mercury (such patients have what is called "low-tension glaucomas"). Thus testing the intraocular tension of a person is not a reliable method for the early detection of glaucoma.
Another proposal for the early detection of glaucoma has involved the assessment of colour vision defects. Simple tests of colour vision defects, however, have shown a lack of correlation between the defects noted and the presence of optic disc cupping. More complex tests of colour vision defects involving anomaloscopy are too difficult for clinic use. Moreover, these tests cannot differentiate between colour vision defects caused by glaucoma and colour deficits resulting from amblyopia and optic neuritis. In addition, it has been reported that up to 25 percent of patients who have glaucomatous scotoma exhibit no colour deficit. Thus assessment of colour deficits in a person's vision is not a reliable method of detecting glaucoma in its early stages, even if it should become practical to perform detailed tests clinically.